In the world of bluegrass, Tim O'Brien is a major player, with several acclaimed albums and a Grammy Award for the 2005 disc "Fiddler's Green." The singer-songwriter has also flirted with mainstream success, working on Dierks Bentley's bluegrass project (2010's "Up on the Ridge") and scoring a Top 10 country hit with Kathy Mattea on 1990's "The Battle Hymn of Love."